A great many types of animal exposure chambers are known in the prior art. In some, the chamber is divided into compartments with individual air supply to each. This type structure requires relatively complicated piping and it is difficult to assure that all animals receive uniform exposure.
In a common type, a plurality of cages are positioned in an undivided chamber, in several layers, and there is a unitary air supply. In such chambers animal excreta catch pans beneath the cages presented a barrier which tended to prevent uniform delivery of the atmosphere to all animals. Either catch pans were omitted, subjecting animals in the lower cages to the excreta of those above, or the chambers were made very large in relation to the size of the cages and catch pans, or internal fans were provided in an attempt to secure uniform distribution. To avoid the problem of uniform distribution and the problem of stress on the lower animals, most exposure systems and chambers are based on research related to one layer of animals being exposed. When cages are stacked, it has been considered necessary to rotate the animals by moving the lowermost cages to the top from time to time.
For an extensive discussion of exposure chambers, see Robert T. Drew and Sydney Laskin "Environmental Exposure Chambers", pages 1 through 41 of Volume 11 of "Methods of Animal Experimentation", edited by William I. Gray, published 1973 by Academic Press. See also R. Carpenter, "Cones, Cone Angles, Plenums and Manifolds", Workshop on Inhalation Technology, Brookhaven National Laboratory, Oct. 16-17, 1978.